A war band would say that...
Lol I'm just joking. I was thinking about picking her up but looking at the feedback regarding her it's kind of a hit or miss. I think what everyone is trying to say is that she does what everyone else does just much less of it. Sure, a heat seeking slow is cool, but why not just play Pyke or Bard? Sure her heal is cool but Sona and Nami already do that. A shield is pretty neat but Braum and Rakan arguably do it better. And I suppose a team root is pretty cool but Maokai and even Neeko's E -> R combo does it better. I'm all down for attaching being a cool new gimmick but what does she really bring that is different to the table? If her Q is so cool, why does it do so little damage? And if her E is ok, why not give her a steroid that no other support has, like increasing health regeneration or healing received? Or maybe when you attach to your ADC they not only gain a shield but an empowered attack. I mean heck, Braum makes an ally's attack have the possibility of stunning an enemy. If Yuumi is so ally dependent, why not transfer the power of 2 champions into one champion?
What I mean by that is the biggest critique with her concerns her early game. Bot lane becomes a 1v2 until late game and essentially, like you said, you're just an item for your adc. Sure you can make some plays by slowing an enemy with your Q but unless you're on a Vayne your adc is probably going to get hooked (and if it's a Pyke level 6 all he needs to do is get within execute range, limiting your E's outplay), rooted, or stunned and then killed. The enemy team adc pretty much needs to 1v1 your adc and you have to hope the enemy support is either incompetent or weak for you to get a kill.
Better yet, her whole thing is being able to open magical portals right? What if her Q had a trail behind it that teleported the enemy to the end of the tail when it hit them, essentially being a heat seeking hook? And then you could make W attach to enemies for a second or two and have it function as a silence or stun so that the enemy support can't just simply follow up and kill your adc while the enemy bot lane laughs at you. She has to have some way of counteracting what most enemy supports do other than just healing. I mean Soraka even has a silence that can root.
I mean you could really have fun with this concept. My favorite attaching champion from a MOBA has been IO from Dota 2, and IO was able to consume items and his ally would also consume some of the item if IO was attached to them. But IO also had Aurelion Sol's stars and could function as a champion.
I think the problem is people don't know what Yuumi is supposed to be. If she's like Xerath in that she plays around her spells and auto attacks to get mana, why is it her spells do so little damage and so little in terms of overall use? And if she's supposed to go from one champion to the next and be a late game monster, what's the point of playing a 4v5 game until you can start actually doing stuff later on? I'm sure the pros will figure out how to use her to win games, but If she's just that kind of champion I can't see her being picked up in casual play as much.
: goes afk on 7/0 vayne
SHUT DOWN 
